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n-Octane aromatization over Pt/KL of varying morphology and channel lengths

Applied Catalysis A General
Publication Date
DOI: 10.1016/j.apcata.2006.07.020
  • N-Octane Aromatization
  • Pt/Kl Zeolite
  • Vapor Phase Impregnation
  • Drifts
  • Tpo
  • Physics


Abstract Various Pt/KL catalysts were prepared over a series of zeolites with different crystallite size and shape. To control the crystallite size and shape and consequently the channel lengths, different zeolite synthesis parameters were varied. X-ray diffraction (XRD) analysis of the synthesized KL zeolites indicated a high degree of crystallinity in all samples. The crystallite morphologies and the length/diameter (L/D) ratios in each sample were characterized by scanning electron microscopy (SEM). The Pt/KL catalysts were prepared by loading 1 wt% of Pt on these supports, using the vapor phase impregnation (VPI) method. They were characterized by diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) of adsorbed CO, volumetric hydrogen chemisorption, and transmission electron microscopy (TEM). It was found that KL zeolites with cylindrical shape are effective catalyst supports for n-octane aromatization, but the effectiveness strongly depends on the channel length of the zeolite crystallite as well as the dispersion and location of the Pt clusters. The different Pt/KL catalysts in the series were compared in the aromatization of n-octane at 500 °C and atmospheric pressure. It was found that the catalysts with shorter channel length exhibited improved activity, selectivity, and catalyst life. Some of these differences can be ascribed to a different distribution of Pt clusters and different metal dispersions. That is, while the cylindrical-shape KL zeolites with short channels (i.e., cylindrical crystallites with low L/D ratio) favored a high dispersion of Pt inside the zeolite, those with longer channels (i.e., high L/D ratio) resulted in a large fraction of Pt cluster outside the zeolite. By contrast, KL zeolites synthesized with nanosize crystallites and having a high external-to-internal surface area ratio yield much lower Pt dispersions and exhibited a large fraction of Pt clusters located outside the pores, which can explain the differences in activity and selectivity. On the other hand, differences in aromatic product distribution observed for the different morphologies are related to a varying extent of secondary reactions. That is, for a given conversion level, the catalysts with shorter channels had a much lower extent of secondary hydrogenolysis. Consequently, more C8-aromatics are preserved in the product and less benzene and toluene are produced compared to the catalysts with longer channels.

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